It has been recognized in the prior art that cellulosic fibers which have been treated to provide cross-linking will no longer inter-link in an aqueous medium, i.e., undergo hydrogen or hydrate bonding. This is in contradistinction to the nature of non-cross-linked fibers which in an aqueous medium undergo cross-linking through hydrogen or hydrate bonding. Accordingly, heretofore cross-linked cellulosic fibers have been used primarily as fillers particularly for sanitary items such as diapers, napkins, tampons, medical dressing, etc., where an improvement in fluid absorptivity is desired, but wherein the need for improved strength, etc., obtained through inter-fiber bonding is not critical. Applications for cross-linked cellulosic fibers are described, for example, in U.S. Pat. No. 3,241,553.
Attempts have also been made according to other disclosures to use cross-linked cellulosic fibers to improve fibrous structures including through the utilization of non-cross-linked cellulosic fibers in combination with cross-linked fibers. Note, for example, French Pat. No. 1,235,963 which discloses the improvement of certain papers, such as filtering and absorbing papers. However, all of the examples relate to the utilization of cotton linters and not wood fibers. The described methods are inapplicable to wood fiber based compositions made by a wet process. Moreover, according to the process of the aforesaid French patent, the properties desired are an increase in porosity and absorption with a compromise in the uniformity of the formed sheet and, hence, its strength. There is no disclosure with respect to paper structures having increased flexibility and improved touch and feel characteristics.
French Pat. No. 1,600,269 discloses a manufacturing process for absorbing papers comprising the cross-linking of part of the fibrous material. Cross-linking takes place after the sheet has been passed over the Yankee drum, whereby the fibers are interlinked and the crepe nature of the paper, according to the described process, is maintained. The fibers are impregnated, then cross-linked, next washed to eliminate the catalyst, large amounts of water being used. An aging stage is required. Cross-linking during processing on the papermaking machine prohibits high industrial efficiency since a high-efficiency papermaking machine operating at about 1,000 meters a minute and a cross-linking phase lasting about 7 seconds would require a heating means over a length exceeding 100 meters.
U.S. Pat. No. 3,455,778 describes a process whereby cotton wool is made by the wet process using a mixture of cross-linked and non-crosslinked fibers. Urea-formol, which is expensive, is used as the cross-linking agent and the need for an impregnation stage prohibits making a low-cost paper. Furthermore, the wet grinding used in this process prohibits total fiber individualization, but on the contrary produces packs or "nodules" of cross-linked fibers "adhering" to each other, whereby machine operation is hampered and sheet formation is impeded, for example by clogging of the pumps by the nodules, etc. Additionally, the cross-linking process used does not provide sufficient output to feed a papermaking machine.